The present invention relates to an underwater fluid transport installation for transporting hydrocarbons for example, and to a method for positioning such an installation between a sea bed and a sea surface situated vertically above the sea bed.
Known installations make it possible to extract the hydrocarbons from underwater deposits. In addition to the difficulties associated with the pressure exerted by the marine environment on these installations, which increases as the depth of extraction increases, other difficulties result from the difference of the disturbances between the sea surface and the sea bed. Specifically, at the sea surface, the water is relatively turbulent to a variable depth of approximately thirty meters beneath the surface, while on the sea bed, it is much less turbulent because it does not sustain the influence of the wind and of the swell in particular. Therefore, such phenomena make it necessary to adapt the installations for extracting the hydrocarbons and for transporting them without disruption from the sea bed to the surface. Therefore, the installations comprise a riser, usually rigid, which extends between the sea bed and a subsurface zone situated beneath the sea surface, and more particularly beneath the aforementioned turbulent zone. This riser is fitted with one or more retaining floats which are installed around it, up to its end in order to keep it stretched in suspension vertically between the sea bed and the subsurface zone. Usually, these retaining floats are cylindrically symmetrical, and the riser passes through them axially. Therefore, the riser is held vertically in a relatively calm zone and its top end is then connected to a flexible duct which leads to a surface vessel floating on the sea surface. In this way, the flexible duct sustains the surface turbulence by deforming without being damaged.
Document US 200700 44 972 describes a system of the hybrid tower type in which one or more floats are mounted around the riser. These retaining floats are symmetrically cylindrical and the riser passes through them axially. However, this configuration is not satisfactory because considerable forces are exerted at the interface between the buoys and the riser, thus weakening the underwater installation.
Many hydrocarbon deposits are situated underground beneath sea beds that are relatively deep, for example more than 1500 meters, and the risers are consequently of increasing length. Therefore, they are increasingly heavy and the floats necessary to hold them in position vertically must be increasingly voluminous in order to increase their buoyancy. Therefore, bringing such floats in line with the hydrocarbon deposits is relatively difficult and requires a large amount of energy since they have to be towed.
Therefore, one problem that arises and that the present invention aims to solve is to propose an installation for the underwater transport of fluids, and precisely of hydrocarbons, which not only makes it possible to extract the hydrocarbons from relatively deep deposits but also which can easily be applied at an advantageous cost and has an acceptable behavior in terms of ageing and fatigue under the effect of the currents and the movements of swells.